Cardiac cine imaging is routinely performed clinically using balanced steady-state free precession (bSSFP) (Finn et al., 2006, Radiology 241(2):338-354) (i.e TrueFISP, b-FFE and FIESTA) due to its high blood-myocardium contrast-to-noise ratio (CNR) and high signal-to-noise ratio (SNR) efficiency. The blood-myocardium CNR for spoiled gradient echo imaging technique strongly depends on inflow, which can result in low blood-myocardium CNR, especially in patients with impaired cardiac function. The SNR in bSSFP is dependent on the T2*/T1 values of the tissues. Due to the inherent differences in T2*/T1 values between myocardium and blood, bSSFP imaging provides higher CNR, even for patients with impaired myocardial function. The bSSFP blood SNR also increases with increasing flip angle (FA) (Markl and Pelc, 2004, J Magn Reson Imaging 20(4):697-705) due to the dependence of the blood signal on in-flow velocity and out of slice effects (Markl and Pelc, 2004, J Magn Reson Imaging 20(4):697-705). The use of higher FAs with a short TR, however, results in substantially increased specific absorption rate (SAR; W/kg), which limits achievable SNR and CNR and can restrict the use of bSSFP for applications such as imaging patients with implanted devices (e.g. pacemakers and ICDs) and imaging at higher field strengths (≥3 T).
Variable Flip Angle (VFA) imaging approaches have been developed to lower SAR for spin echo imaging (Hennig et al., 2003, Magn Reson Med 49(3):527-535; Busse et al., 2006, Magn Reson Med 55(5):1030-1037), to obtain T2-weighted contrast with bSSFP (Paul et al., 2006, Magn Reson Med 56(1):82-93) or SSFP-echo (Srinivasan et al., 2012, In Proceedings of 20th Annual Meeting of ISMRM, Melbourne, Australia; 290.) sequences, to increase spatial resolution for bSSFP imaging (Worters and Hargreaves, 2010, Magn Reson Med 64(5):1404-1412); and to increase SNR in bSSFP (Paul and Zaitsev, 2009, Magn Reson Imaging 27(7):933-941) and spoiled gradient echo (Stehling, 1992, Magn Reson Imaging 10(1):165-167) non-cine imaging. All of these techniques have used single-shot or multi-shot imaging, but without application to cardiac cine imaging, which is conventionally acquired using segmented k-space acquisitions.
Conventional k-space segmented cardiac cine bSSFP images (FIG. 1A) acquire the first k-space segment for each cardiac phase during the first cardiac cycle (R-R interval) followed by the acquisition of the second segment for each cardiac phase during the second cardiac cycle and so on until all k-space segments are acquired. However, various VFA schemes coupled to segmented k-space acquisitions and bSSFP are not able to reduce the overall SAR of the sequence efficiently and produce significant image artifacts. Thus, there is a need in the art for an improved system and method for lower SAR or higher contrast cardiac cine imaging. The present invention satisfies this need.